Field of the Invention
The present invention relates to an anti-reflection film which is provided on an optical surface of an optical element main-body to reduce reflection of incident light, and a method for manufacturing the anti-reflection film and more particularly relates to an anti-reflection film comprising a concave-convex nanostructure, and a method for manufacturing the anti-reflection film.
Background Art
In a conventional optical element such as a lens, an anti-reflection film comprising an anti-reflection structural body is provided on an optical surface to reduce a loss of transmitting light due to surface reflection. A concave-convex nanostructure in which convex parts are regularly arranged at a shorter interval than the wavelength of incident light has been known as one example of the anti-reflection structural body (e.g., see “Patent document 1: National Publication of International Patent Application No. 2010-511079” and “Patent document 2: Japanese Patent Laid-Open No. 2010-48896”). When the concave-convex nanostructure is provided on an optical surface of an optical element, an anti-reflection effect can be achieved in a wide wavelength range and a wide incident angle of an incident light.
The concave-convex nanostructure has numerous convex parts that protrude from the optical surface side of the optical element. To reduce reflection, it is necessary to form a gradual refractive index distribution in a depth direction of the concave-convex nanostructure from the air as a medium of incident light. For this reason, the convex part may have a tapered shape whose peak is thinner than its proximal end. Thus, a problem that the surface of the concave-convex nanostructure might be easily damaged mechanically, i.e. poor in scratch resistance.
The surface area of the concave-convex nanostructure is much larger than the surface area of the optical surface of the optical element. Thus, for example, when the concave-convex nanostructure is kept for a long period of time under a high-temperature and high-humidity environment, water or the like adsorbs to the concave-convex nanostructure and the spoiled concave-convex nanostructure may cause a problem, poor anti-reflection performance.
To solve the problems, for example, the anti-reflection film disclosed in Patent document 1 prevents mechanical damage or water adsorption by providing a transparent protective coat having a film thickness of 10 nm to 50 nm on the surface of the concave-convex nanostructure.
However, in the protective coat disclosed in Patent document 1, concave parts of the concave-convex nanostructure are filled with a cover layer constituting material. Thus, the gradual refractive index distribution formed in the depth direction of the concave-convex nanostructure from the air as the medium of incident light cannot be maintained, and the anti-reflection performance of the anti-reflection film may be made poor.
In the anti-reflection film comprising a refractive index gradient layer, a refractive index distribution ideal in reducing the reflection has been known (e.g., see Non Patent document 1: “Practical Design of Optical Thin Films” by Ronald R. Willey, p. 101, Willey Optical, Consultants, November 2007) (see FIG. 2). However, it is difficult to form the ideal refractive index distribution in the depth direction just by the concave-convex nanostructure, and further improvement in the anti-reflection performance is required.
In view of the above, an object of the present invention is to provide an anti-reflection film comprising a concave-convex nanostructure excellent in both high-temperature and high-humidity environment resistance and scratch resistance in addition to further improved anti-reflection performance, an anti-reflection optical element comprising the anti-reflection film, and a method for manufacturing the anti-reflection film.